Water and pollution
After removing some effects generated by the local atmosphere, the researchers examined the spectrum of colors to see what molecules were visible. A few surprises popped up.
For example, they didn't see as much water vapor in the signature as observers saw in a 2009 eclipse that encompassed much of the Northern Hemisphere. (That paper, "Earth’s transmission spectrum from lunar eclipse observations," was published in the journal Nature.)
Researchers in the newer study concluded that the absence of water vapor was because the "path" of the 2011 transit in the Earth's atmosphere included the Antarctic, where much of the water is presumed to be frozen out of the atmosphere.
Another surprise was the abundance of nitrogen dioxide. Normally the nitrogen dioxide is regarded as a pollutant produced by human activities. The Antarctic, however, is quite a barren location — but it did have a volcano.
"We found that the track we observed is close to a volcano, and this volcano can potentially produce nitrogen dioxide," Yan said. [The Biggest Volcanoes on Earth]
He added that other explanations could be possible. In this case, the volcano (Mount Erabus) may not be active enough to produce large amount of nitrogen dioxide. Further investigation found that the nitrogen dioxide was a bio-product of nitrous oxide (which is produced naturally by microbes) that then lingered in the atmosphere and reacted with ozone, creating nitrogen dioxide.
"This was during the spring, and the ice melted in the spring, and according to the vulcanologists this melt releases a lot of nitrous oxide," Fosbury said.
Ozone on other planets
If we were to look at Earth as an exoplanet, could the nitrogen dioxide be interpreted as a sign of pollution, of microbial life or of a volcano? Fosbury said it depends on context. If the planet had an abundance of volcanoes on its surface, you would assume it was likely, principally, from the volcanoes. If those weren't easily visible, it would be harder to draw conclusions about life, but it would be possible.
He pointed out that nitrogen dioxide is normally associated with pollution.
"It's over Los Angeles and Beijing and all of those places because of how the catalysis of exhaust [from cars] works," Fosbury said.
When looking for an extraterrestrial civilization, pollution chemicals should be included on the list of "signs" of life, he added.
Ozone might also be visible. Fosbury pointed out that at higher latitudes, at the edge of the umbra on the moon, you can see blue.
"It's one of the indicators that there's a lot of ozone," he said.
Ozone is also the reason that the sky looks blue during twilight at dusk dawn. (Daylight blue is because of a phenomenon called Rayleigh scattering, which preferentially scatters blue light from the sun through the air.)
"Ozone actually is a very prominent and very important marker for Earth-like planets," Fosbury said.
ESO, whose astronomical facilities are based in Chile, also has at least two major contributions to exoplanet research. The High Accuracy Radial velocity Planet Searcher (HARPS) at the ESO La Silla 3.6-meter telescope measures small variations in stellar velocities as planets orbit them. This instrument was used for the first-ever detection of an exoplanet.
Also under construction is the European Extremely Large Telescope (E-ELT), a 39-meter beast that will not only do these velocity measurements, but also image some planets and possibly characterize their atmospheres. This research will come in handy when the E-ELT and NASA's James Webb Space Telescope are working.
"This will be quite an investment over a long period of time," Fosbury added. "As we learn more about the practicalities of doing these observations, we'll be in a better position to not only perform the observations, but design the kinds of instruments that will be needed."